D R A F T - ASCE-ASABE PROPOSED CN Update, September 30, 2017 1 United States Department of Agriculture 2 Natural Resources Conservation Service 3 Part 630 Hydrology 4 National Engineering Handbook 5 Chapter 9: Hydrologic Soil-Cover Complexes 6 Acknowledgements 7 Chapter 9 was originally prepared by Victor Mockus, (deceased) hydraulic engineer, USDA Soil 8 Conservation Service, and was published in 1964. It was reprinted with minor revisions in 1969. This 9 version was prepared by the Natural Resources Conservation Service (NRCS)/Agricultural Research 10 Service (ARS) Curve Number Work Group and Helen Fox Moody, retired hydraulic engineer, 11 NRCS, Beltsville, Maryland. 12 A previous revision was prepared by the Natural Resources Conservation Service (NRCS)/Agricultural 13 Research Service (ARS) Curve Number Work Group. 14 This September 2017 revision is based on the publication originally developed by that work group, which 15 was composed of scientists and engineers from: 16 Natural Resources Conservation Service - Donald E. Woodward, Robert D. Nielsen, Robert Kluth, 17 Arlis Plummer, Joe Van Mullem, and Gary Conaway; 18 Agricultural Research Service - William J. Gburek, Keith Cooley, Allen T. Hjelmfelt, Jr., and Virginia 19 A. Ferriera; and 20 University of Arizona - Richard H. Hawkins. 21 The authors and contributors to this September 2017 revision are indebted to their original work. Chapter 9, September 2017 Revision 1 D R A F T - ASCE-ASABE PROPOSED CN Update, September 30, 2017 22 Authors and Contributors: This September 2017 revision was prepared by the Curve Number Task 23 Group of the Watershed Management Technical Committee, Environmental Water Resources Institute 24 (EWRI) of the American Society of Civil Engineers. The major authors and contributors have been, in 25 alphabetical order: Hunter Birckhead, P.E., M.ASCE; James V. Bonta, Ph.D., P.E., F.ASCE; Donald 26 Frevert, Ph.D., P.E., D.WRE(Ret), F.ASCE; Claudia Hoeft, P.E., F.ASCE (USDA NRCS liaison); Richard 27 H. Hawkins, Ph.D., P.E., F.EWRI, F.ASCE (Task Group chair); Rosanna La Plante, P.E., M.ASCE; 28 Michael E. Meadows, Ph.D., P.E., F.ASCE; Julianne Miller, A.M.ASCE; Steven C. McCutcheon, Ph.D., 29 P.E., D.WRE(Ret), F.EWRI, F.ASCE; Glenn Moglen, Ph.D., P.E., F.EWRI, F.ASCE; David Powers, P.E., 30 D.WRE, F.ASCE; John Ramirez-Avila, Ph.D., ING., M.ASCE; E. William Tollner, Ph.D., P.E., M.ASCE, 31 F.ASABE (American Society of Agricultural and Biological Engineers [ASABE] representative), Joseph 32 A. Van Mullem, P.E., M.ASCE; Tim J. Ward, Ph.D., P.E., F.EWRI, F.ASCE (Task Group co-chair),; and 33 Donald E. Woodward, P.E., F.ASCE (Task Group co-chair). 34 35 Contents 36 630.0900 General ............................................................................................................................ 5 37 630.0901 Determinations of complexes and Curve Numbers ........................................................ 5 38 (a) Agricultural land ................................................................................................................... 5 39 (1) Historic assignment of CNs to complexes ......................................................................... 5 40 (2) Use of Table 9–1 ................................................................................................................ 6 41 (b) National and commercial forest: forest-range ...................................................................... 9 42 (1) Forest-range in Western United States ............................................................................... 9 43 (c) Urban and residential land .................................................................................................. 13 44 (1) Connected impervious areas ............................................................................................. 13 45 (2) Unconnected impervious areas ......................................................................................... 15 46 (d) Karst Hydrology and the CN Method.................................................................................. 17 47 630.0902 Curve Number Variation with Slope ............................................................................ 18 Chapter 9, September 2017 Revision 2 D R A F T - ASCE-ASABE PROPOSED CN Update, September 30, 2017 48 630.0903 Curve Number Variation with Season .......................................................................... 21 49 630.0904 Regional Variation ........................................................................................................ 22 50 630.0905 Drainage Area Limitations ............................................................................................ 22 51 630.0906 Local Information Tables .............................................................................................. 23 52 630.0907 Examples ....................................................................................................................... 24 53 630.0908 Appendices .................................................................................................................... 26 54 Appendix 1 – Suggested Curve Number Assignments for the National Land Cover Database 55 (NLCD): Ia/S = 0.05 Basis ........................................................................................................ 26 56 Appendix 2 - Determination of Curve Numbers from Data ..................................................... 31 57 630.0909 References ..................................................................................................................... 47 58 59 Tables 60 Table 9- 1. Runoff Curve Numbers for agricultural lands1 ............................................................ 6 61 Table 9- 2. Runoff Curve Numbers for arid and semiarid rangelands1 .......................................... 9 62 Table 9- 3. Limitations on the use of Curve Numbers in forests ................................................. 12 63 Table 9- 4. Curve Numbers for urban conditions1 ....................................................................... 14 64 Table 9- 5. Curve Numbers for green roofs ................................................................................. 17 65 Table 9- 6. Curve Numbers for permeable pavement over HSG subbases ................................. 17 66 Table 9- 7. Summary of slope effects on CN............................................................................... 19 67 68 Table 9A- 1. NLCD land cover classes, descriptions, and associated CNs. ................................ 27 69 Table 9A- 2. Data for fitted CNs for selected illustrative cases, natural data case ...................... 41 70 Table 9A- 3. Data for fitted CNs for selected illustrative cases, rank-ordered (asymptotic) case41 71 Table 9A- 4. Violent response summary for CN∞ ....................................................................... 44 Chapter 9, September 2017 Revision 3 D R A F T - ASCE-ASABE PROPOSED CN Update, September 30, 2017 72 73 Figures 74 Figures 9A-1 a and 9A-1b. Rainfall (P) - Runoff (Q) and Complacent Curve Numbers for West 75 Donaldson Creek, Oregon ...........................................................................36 76 77 Figure 9A-2 a. CN determination for plot CL4, Jornada Range, New Mexico. .......................... 38 78 Figure 9A-2 b. CN determination for watershed 26020, Coshocton, Ohio.. ............................... 39 79 Figure 9A-2 c. CN determination for Safford watershed 4, Arizona. ........................................ 39 80 Figure 9A-2 d. CN determination for Edwardsville watershed 2, Illinois.. ................................. 40 81 Figure 9A-3. Berea Watershed 6, Kentucky 43 82 Chapter 9, September 2017 Revision 4 D R A F T - ASCE-ASABE PROPOSED CN Update, September 30, 2017 83 630.0900 General 84 A combination of a hydrologic soil group (soil) and a land use and treatment class (cover) is a 85 hydrologic soil-cover complex that defines a Curve Number (CN). This chapter provides tables 86 and graphs of runoff curve numbers (CNs) assigned to such complexes. The CN indicates the 87 runoff potential of a complex during periods when the soil is not frozen or there is no snow on 88 the ground. CNs are used to estimate runoff from rainfall, only. A higher CN indicates a higher 89 runoff potential and specifies which runoff curve or Figure 10–2 in National Engineering 90 Handbook, Part 630 (NEH 630 (USDA NRCS (1999)), Chapter 10, is to be used in estimating 91 runoff for the complex. Applications and further description of CNs are given in NEH 630, 92 Chapters 10 and 12. 93 94 630.0901 Determinations of complexes and Curve Numbers 95 (a) Agricultural land 96 Complexes and assigned CNs for combinations of soil groups of NEH 630, Chapter 7 and land 97 use and treatment classes of NEH 630, Chapter 8, are shown in Table 9–1. Impervious surfaces 98 and water surfaces, which are not listed, are always assigned a CN of 97. 99 (1) Historic assignment of CNs to complexes 100 Table 9–1 was initially developed as follows: 101 The data literature was searched for watersheds in single complexes (one soil group and one 102 cover); watersheds were identified for most of the listed complexes. 103 A median CN for each watershed was obtained using rainfall-runoff data for all storms 104 producing the annual peak runoff. The watersheds were generally less than 1 square mile in area, 105 the number of watersheds for a complex varied, and the storms were of one day (24 hours) or 106 less duration.